Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and apparatuses for estimating signatures characterizing signals penetrating the seafloor in shallow water, or more specifically, to methods and apparatuses able to estimate these signatures using near-field measurements without comprehensive knowledge of the water-bottom's depth and reflectivity in the survey area:
Discussion of the Background
In seismic surveying, seismic signals (i.e., pressure variations propagating in an explored volume) are used to investigate geophysical structures under the ground surface or under the seafloor. Seismic data representing reflections of the seismic signals in the investigated geophysical structure are acquired and processed to generate images of the investigated structure. These images may be used to evaluate oil and/or gas reservoirs. Improving seismic data acquisition and processing is an ongoing research topic.
During a marine seismic survey, a submerged seismic source generates signals at different horizontal locations in the surveyed area. The signals propagate in all directions linearly until encountering interfaces where the propagation velocity changes (e.g., at an interface between water and air, water to rock, shale to sand, etc.). At these interfaces, the signals are reflected, refracted and/or transmitted. Some of the signals' energy eventually reaches detectors in streamers towed or placed on the seafloor. The detectors (also known as receivers) are configured to record information (seismic data) related to the source-generated signals that traveled through the investigated geophysical structure.
Each recording in the seismic data is due to one receiver detecting energy after a shot and includes a convolution of the source signature (i.e., amplitude versus time characterizing the signal penetrating the seafloor following the shot) and the investigated structure's response function. The structure's response function associated with a location carries information about the nature and depth of interfaces between layers of the structure under the seafloor. During seismic data processing, the signature of the signal incident on the formation (i.e., penetrating the seafloor) is used to apply a designature procedure to the seismic data in order to extract the structure's response function. The structure response functions extracted from different recordings (and thus corresponding to different locations in the surveyed area) are then used to create the investigated structure's image(s).
Marine seismic sources usually include multiple individual sources that are fired substantially simultaneously to generate a seismic signal stronger than achievable with a single individual source. An individual source may be an air-gun or a cluster of air-guns. The shape of the signal (i.e., amplitude versus time) generated by the source (i.e., due to all the individual sources) varies with distance until, at a great enough distance, it starts having a stable shape. After the signal's shape becomes stable, its overall amplitude decreases inversely proportional to the distance. The region where the signature shape no longer changes significantly with distance is known as the “far-field,” in contrast to the “near-field” region where the shape varies.
For large water-bottom depths, the source signature may be calculated using equivalent notional signatures for the individual sources. The equivalent notional signature is a tool for representing the contribution of an individual source to the signal in the far-field region, with each individual source contribution being decoupled from contributions of the other individual sources. As described in U.S. Pat. No. 4,476,553 and U.S. patent application Ser. No. 2013/0258808 (the entire contents of which are incorporated herewith by reference), equivalent notional signatures may be obtained using the near-field measurements and information about the individual sources' arrangement when the source is fired. The near-field measurements may be acquired by near-field sensors placed in proximity to each individual source (e.g., hydrophones about 1-2 m above each air-gun). The source signature that characterizes the signal actually penetrating the seafloor is a superposition of the notional signatures corresponding to each of the individual sources.
In the case of shallow water (called also “extreme shallow water” and meaning water-bottom depths up to 150 m), calculating notional signatures from the near-field sensor measurements becomes problematic because the near-field sensors also detect water-bottom reflections related to the signals that do not penetrate the seafloor. As exemplarily illustrated in FIG. 1, when air-gun 100 is fired in shallow water, a near-field sensor 110 placed above air-gun 100 detects: (A) an up-going direct (traveling straight from the air-gun to the near-field sensor) signal 120, (B) a water-air interface (i.e., water surface 10) reflection 130, (C) a first water-bottom reflection 140 of a down-going signal traveling from the air-gun to the seafloor 20, (D) a first water-bottom reflection 150 of a signal that has previously been reflected at the water-air interface, (E) a water-air interface reflection 160 of a signal similar to reflection 140, (F) a water-air interface reflection 170 of a signal similar to reflection 150, etc.
In contrast, if the water-bottom is deeper (i.e., not in the shallow water range), the near-field sensor detects only the up-going direct signal 120 and the water-air interface reflection 130. FIG. 2 is a graph of near-field measurements (i.e., amplitude versus time) for the same air-gun. Continuous line 210 is the near-field measurement for water-bottom deeper than 150 m. Dashed line 220 corresponds to a simulated water-bottom reflection such as 140 in FIG. 1. Dash double point line 230 corresponds to a simulated reflection such as 150 in FIG. 1. Line 240 is the measurement acquired with the near-field sensor for water-bottom depth in the shallow range. Line 240 is a sum of the values on lines 210, 220 and 230, and overlaps line 210 except for the time interval between 0.13 and 0.16 ms. Since the water-bottom reflections do not penetrate the seafloor, the near-field measurements in the shallow range are unsuitable for determining the signature usable in the seismic data processing.
An attempt to overcome this problem has been made by including the water-bottom reflections in the propagation model (as described in “Source signature estimation—Attenuation of the seafloor reflection error in shallow water,” by J-F. Hopperstad and R. Laws, presented at EAGE 68th Conference & Exhibition, Vienna, Austria 12-15 Jun. 2006). However, this method requires precise knowledge of the water-bottom's depth and reflection coefficient over the surveyed area. These parameters with the required precision are not usually available throughout a complex surveyed area. Moreover, the water-bottom's reflection coefficient may not always be described as a scalar. Therefore, the propagation time to and from the water-bottom and amplitudes of water-bottom reflections may not be evaluated correctly using this approach.
Accordingly, it would be desirable to develop methods and apparatuses for determining the signature of the signal penetrating the seafloor during seismic surveys in shallow water.